1. Field of the Invention
The present invention relates to the field of silica crucibles and more particularly to a silica crucible having a multi-layer wall with a bubble-free inner layer that is subject to very little bubble growth when holding molten silicon and an outer layer that is opaque and stable.
2. Background of the Invention
Silicon wafers used in the semiconductor industry are made from ingots of single crystalline silicon. Such ingots generally are manufactured using the Czochralski (CZ) process. In the CZ process, metallic silicon is charged in a silica glass crucible housed within a susceptor that is received in a crystal growth chamber. A heater surrounding the susceptor heats the charge thus melting the silicon. A single silicon crystal is pulled from the silicon melt at or near the melting temperature of silicon.
At operating temperatures, the innermost layer of a silica crucible reacts with the silicon melt by dissolving into the silicon melt during the CZ-process. Any bubbles in the inner layer tend to open to the melt during such dissolution. This is a source of particles in the melt, which may disturb the single crystal structure of the pulled ingot. A bubble-free inner layer has been a critical requirement for a crucible adapted for the CZ process.
Recently, demand in the semiconductor industry is for large diameter wafers, e.g., 200 mm or 300 mm in diameter. Pulling multiple ingots by recharging silicon after each pull has became a popular modification of the traditional CZ process. As a result, the CZ process operating period is increased, frequently to more than one hundred hours. This CZ process means that the thickness of the bubble-free layer must also be increased. In the case of a crucible used for 150 mm-diameter wafer production, a crucible having bubble-free inner layer thickness of one millimeter was sufficient. It is not uncommon, however, to dissolve more than two millimeters of a silica crucible inner layer when pulling 200 mm or 300 mm diameter ingots.
In addition, the longer operating time means that bubble growth in the inner layer during the CZ process must be minimal. A miniscule bubble in a fused crucible may grow in size during operation. Larger bubbles are prone to release larger particles, causing potentially greater harm to the ingot. When the CZ process proceeds for long times, evolution of bubbles is routinely observed. These growing bubbles result from gas that is dissolved in the inner layer and may be essentially invisible after the crucible is formed. Minimizing evolution and growth of bubbles is very important for crucibles used in modern CZ process applications.
Although it is important to minimize bubbles and bubble growth in the inner layer, it is well known that the outer layer of a multi-layer crucible used for the CZ process must include bubbles to make the outer layer opaque. This creates a layer that diffuses thermal radiation. The importance of the mechanical stiffness and integrity of the crucible sidewall has been recently identified for large crucibles, such as those of 550 mm diameter (nominal 22-inch) or greater. We have found that bubble growth in the opaque layer results in a swelling of the crucible wall. During the CZ process the crucible resides in a rigid graphite susceptor. It nonetheless experiences an increase in wall volume. This volume change causes a serious quality problem in the pulled ingot, especially when the silicon melt level movement is relatively low or the ingot pulling speed is slow. But all other things being equal, slow pulling produces high quality ingots. As a result, it is desirable to prevent wall volume changes to permit slow pulling speeds.